Preventative Measures: Eradicating Rats

By Amanda Murek and Dalton Fountain

In comparison to other cities of the late 19th and early 20th century, London needed strong precautions because of its high amounts of importing trade. At this time plague deaths were being reported in Hong Kong, India and Los Angeles, and Australia was experiencing an outbreak for the first time. London took notice and implemented the search for and examination of rats, the destruction and prevention of rat infestation, the certification of rat-inspected ships, and the monitoring of port and ship human health. The extensive precautions taken in the Port of London as described were in good reason proportionate to the devastating effects of plague that are well-known throughout history.

As the Third Plague Pandemic began in the late 19th century, large cities like London began rigorous measures to keep it out. People in the 20th century understood that rats and plague were linked, because rats carried fleas which in turn carried Yersinia pestis, the bacteria that causes plague. (Ecology and Transmission). The emphasis of plague transmission was on rats because the fleas that live on rats are the most common carriers of the plague bacteria, Yersinia pestis. When an infected flea bites an animal, being a rat, squirrel, dog, or human, the plague bacteria is transmitted. The prevention of plague in 20th century London consisted of the search for and examination of rats, the destruction and prevention of rat infestation, the certification of rat-inspected ships, and the monitoring of port and ship human health. 

“Rats roaming the sewers, some of them dying, heralding the plague”
Wikimedia Common

An MOH report from 1910 states that rats are the source of the plague and that they must all be destroyed (Poplar 1910). While in another report from the same year they talk about how they came to recognize that it was not the rats that were carrying the plague but the fleas on them and gives examples to prove it (Stoke Newington 1910). In the reports we can see that they even knew that the fleas carried the bacteria that caused the plague as well as what caused them to get the plague (Acton 1901). Because they recognized the role of rats in spreading the plague, they worked to eradicate rats in mass numbers in attempts to prevent the plague from appearing (Port of London 1912).

“A rat stowing away on a ship, carrying the plague further”
Wikimedia Common

Another way that London prevented the spread of plague was in the capturing and examination of rats in ports and on ships. In the reports we can see that they exterminated rats whenever they found them on ship out of fear of them bring or spreading the plague (City of London 1919). Towns and ports took great precautions against the rats and the risk of them spreading the plague. One report describes how medical officers would board the ship and take into account the amount of rats and how many were infected, even marking the number of each color rat to try and see if there was a correlation between a certain rats’ fur color and them carrying the plague (City of London 1930).

A long-term organization that specialized in the trapping and examination of ship rats implemented “rat-searchers” to look for dead rats as well as capture any live rats. In the case of examining ships from plague-infected ports, the rat-searchers were to visit the ship every day that it was in port, looking for any dead rats that may have been exposed due to the discharge of cargo. The rat-searcher also is assigned to examine the ports and shores for signs of rats. To logistically do this, they divide the entire district into sections on a map and search the district by section over the course of a week.

Warehouse keepers were requested to make extensive attempts to trap rats over the week and the rat-searcher would pick them up each day to be microscopically examined. The medical officers of the period even tried to capture rats for study of the disease and sent letters to mayors asking them to inform him if they found any neighborhoods infested (Poplar 1911). The Seamen’s Hospital in Greenwich, UK had a lab in which qualified bacteriologists examine the rats. The rat searchers tied labels to the leg of each rat, giving it a number and area of port from which it came. The rat-searchers also used a daily logbook in which they wrote the tag information along with notes on how the rat was obtained. They put the rats into a linen bag and then into a tin box which is delivered to the laboratory at the end of the day. The bags were dipped in kerosene at the laboratory to kill any fleas before reuse. If any rats were found to have plague bacteria, the Medical Officer was notified and they used the rat-searcher’s reports to locate exactly where the rat was obtained to carry out precautions and disinfection at that location. If a rat-searcher found a dead rat with obvious signs of Plague, it was taken to the laboratory immediately for bacteriological examination and a report was made to the Medical Officer.  These measures were taken to ensure the fastest detection and disposal of plague before spread to the city.

Another way in which plague was prevented in London was simply the extermination of rat infestations, called deratisation. Larger rat populations would more easily catch and spread plague than smaller rat populations, so decreasing the numbers of rats is a preventative measure in itself. Attempts were made to exterminate ship rats before the discharge of cargo to port and traps were kept in all areas of ships that were prone to rat infestation. Ship workers were advised to catch or destroy any rats as possible, both while on voyage and while in port. Fumigation was a means of rat extermination and chemical gases such as Hydrogen Cyanide, Sulfur Dioxide, sodium fluoroacetate, and Warfarin were used. In addition, rats were prevented from getting to shore from foreign ships. Ropes used to secure ships in port were covered in tar each night and fitted with rat guards such as metal brushes or funnels to prevent the passage of rats to shore. All empty barrels and containers from ships were examined for rats before brought ashore. Rat populations were kept as low as possible and attempts to protect the mainland from rat invasion were enforced.

A certificate of ship deratisation was issued as credentialing for which ships were least likely to spread plague. Every six months, inspection of ships was performed by officers of the Port Sanitary Authority to determine the level of rat population on board. Ships with minimal rat infestation may be issued a Certificate of Exemption from Deratisation, and ships that had significant signs of rat infestation must conduct deratisation protocol to receive a Certificate of Deratisation. One of these certificates was required for each ship in order to check in at the London port. If a ship could not show a valid Certificate, an officer of the Port of Sanitary Authority must come aboard to inspect the ship. In the case of intense risk of plague transmission, as with a ship from a heavily plagued area, deratisation protocol may be required despite a valid certificate.

The human population of the port and ships were watched for plague sickness as well. Transportation and isolation were made available for any infected person and Medical Officer reports were made to keep track of the health of the passengers. Suspects on board were disinsected and sometimes placed under surveillance. When there were suspicions of a plague-sick passenger, they were given isolated medical care and the person’s living area and belongings were disinfected. 

In order to prevent the rats from spreading the plague they made a new job for people to perform: rat searching. Rat searching was where someone would patrol his “district” and would monitor the rats by the labels on their legs they would place to determine if they were infected (Port of London 1929). Furthermore they found ways to tell if a rat was infected by the plague just by their appearance, such as them being dazed, confused and convulsing (London City Council 1900).

The medical officers of the time also had steps that people should follow to dispose of dead rats if they should find the to prevent the chance of it spreading the plague. The method was to pick them up with preferably tongs so as to have no skin touching it then burning the body one sure way to stop the diseases spread (Hanover Square 1900). In one British medical journal it talks about the safety measures they would take on suspected ships to prevent the rats from escaping. they went as far as to placing guards to watch the ropes the ship used to stay at the dock (Shipborne Rats and Plague).

Primary sources

Acton 1901

City of London 1919

City of London 1928

City of London 1930

City of London 1936

City of London 1967

City of London 1968

City of London 1972

Ealing 1966

Hanover Square 1900

London County Council 1900

Poplar 1910

Poplar 1911

Port of London 1907

Port of London 1912

Port of London 1929

Stoke Newington 1910

Surbiton 1910

Secondary sources

Benny, Sally. “Timeline.” The History of the Plague.

The Black Death: The Plague, 1331-1770.” John Martin Rare Book Room.

CDC, Plague and its Ecology and Transmission “Shipborne Rats And Plague.” The British Medical Journal 1, no. 2105 (1901): 1100.

Frith, John. “The History of Plague- Part 1. The Three Great Pandemics.” JMVH. History Volume 20 No. 2.

“Shipborne Rats And Plague.” The British Medical Journal 1, no. 2105 (1901): 1100.